One method for separating polypeptides makes use of their...

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends on the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which of each pair of polypeptides that follow is more soluble at the indicated $\mathrm{pH} ?$ (a) (Gly) $_{20}$ or (Glu) $_{20}$ at $\mathrm{pH} 7.0$ (b) (Lys-Ala)$_{3}$ or (Phe-Met) $_{3}$ at $\mathrm{pH} 7.0$ (c) (Ala-Ser-Gly) $_{5}$ or (Asn-Ser-His) $_{5}$ at $\mathrm{pH} 6.0$ (d) (Ala-Asp-Gly) $_{5}$ or (Asn-Ser-His) $_{5}$ at $\mathrm{pH} 3.0$

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends on the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which of each pair of polypeptides that follow is more soluble at the indicated $\mathrm{pH} ?$
(a) (Gly) $_{20}$ or (Glu) $_{20}$ at $\mathrm{pH} 7.0$
(b) (Lys-Ala)$_{3}$ or (Phe-Met) $_{3}$ at $\mathrm{pH} 7.0$
(c) (Ala-Ser-Gly) $_{5}$ or (Asn-Ser-His) $_{5}$ at $\mathrm{pH} 6.0$
(d) (Ala-Asp-Gly) $_{5}$ or (Asn-Ser-His) $_{5}$ at $\mathrm{pH} 3.0$

Key Concepts

pH Dependence of Ionization

The pH of the solution plays a critical role in determining the charge state of amino acid side chains. When the pH is above or below the pKa of a side chain, the ionization (and thus the charge) of that group will change, affecting the net charge of the peptide. These shifts in charge directly influence how well the peptide interacts with water, thereby affecting solubility.

R Group Polarity

Different amino acids have side chains (R groups) that vary in polarity. Polar and charged R groups attract water molecules and enhance solubility, whereas nonpolar or hydrophobic R groups do not interact well with water, reducing solubility. Understanding the relative polarity of the amino acid side chains in a peptide helps predict its solubility in a given environment.

Polypeptide Solubility

This concept relates to the ability of polypeptides to dissolve in a solvent, which in the case of aqueous solutions is largely influenced by the presence of charged and polar groups along the peptide chain. In solubility-based separation methods, peptides with a higher number of ionized functional groups tend to interact more favorably with water, increasing their solubility.

Ionizable Amino Acid Side Chains

Amino acids often contain side chains that can either accept or donate protons depending on the pH of the environment. This ionization alters the overall charge and polarity of polypeptides. The degree of ionization, which is determined by the pKa values of these side groups in relation to the pH, is a crucial factor in determining the solubility of a peptide.

Transcript

00:01 Hey there.

00:03 So our question in this video is about the solubility of peptides, the solubility of various peptides.

00:11 And the trick to this is about looking for the relative polarity of the amino acid side chains.

00:19 So first we'll be looking out for amino acids with charged side chains.

00:26 Then we'll be looking for amino acids with polar side chains.

00:32 Then we're looking at non -polar.

00:34 Side chains.

00:36 So basically the charge side chains are more soluble than the polar side chains.

00:52 So basically this is decreasing solubility.

00:56 So the charge chains, the charge side chains are more soluble than the polar side chains and then the polar side chains are more soluble than the non -polar side chains.

01:10 As you go through this, one of the things that would be very important is to take notice of the ph.

01:18 So basically at ph 7, usually expect all the side chains to be charged.

01:24 Yeah, that's page 7 and just expect your side chains to be charged.

01:45 So yeah, ph 7 can expect of your side chains.

01:47 However, it's also important to also know the p .kr of the side chains.

01:53 So if you have a ph that is greater than if you have, sorry, if you have a ph that is greater than a p .r.

02:06 The ph greater than a pqr.

02:07 Then that pchr will be unprotonated.

02:13 So i'll present as minus h plus, unprotonated.

02:17 But if you have the pqr rather being greater than the ph, then that's pchr.

02:26 P -k -r will be protonated, which are written as plus h -plus, will be puttonated...

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